1. Field of the Invention
The present invention relates to laser printer exposure control, and more specifically relates to a raster scanning apparatus capable of changing picture element density.
2. Description of the Related Arts
Conventional raster scanning apparatus expose/scan using a modulated laser beam with a polygon mirror or the like to form an image on a photosensitive material in accordance with image data. In conventional raster scanning apparatus the photosensitive material used is a silver salt film or silver salt paper or the like. The degree of photosensitivity possessed by the aforesaid photosensitive materials changes according to the type of material used. For example, silver salt films and silver salt paper have sensitivity ratios of about 1:2. (Silver salt paper has about double the sensitivity of silver salt film.) Accordingly, image density will vary for the same raster scanning apparatus depending on the type of photosensitive material used for image forming. Thus, the laser power must be optically or electrically modified to produce same density images on photosensitive materials having different degrees of photosensitivity.
Changing laser power in accordance with the sensitivity of the photosensitive material has the disadvantage of producing influences based on the precision of the laser detecting sensors. That is, the laser detecting sensor cannot monitor a broad range of laser output frequencies with any degree of precision. Further, this lack of broad range monitoring capability leads to cost increases for the apparatus.